Protection of hydrophilic films,layers,and products thereof



United States Patent Office 3,503,743 Patented Mar. 31, 1970 3,503,743 PROTECTION OF HYDROPHILIC FILMS, LAYERS, AND PRODUCTS THEREOF Jaromir V. Kosar, Beechurst, N.Y., assignor to Diagravure Film Mfg. Corp., Brooklyn, N.Y., a corporation of New York No Drawing. Filed May 26, 1966, Ser. No. 553,055

Int. Cl. G03c 1/76, 1/68 US. CI. 9667 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION My invention constitutes improvements in stencils and materials for making stencils and more particularly to improvements relating to the preservation and protection of hydrophilic films or layers, for example, the gelatin films of stencil sheets.

One known method of printing graphic images is the screen process in which a stencil is applied to a surface to be printed and coloring matter such as printing ink is passed through the openings of the stencil onto the surface. Excess coloring matter is wiped off and the stencil is removed, thereby leaving on the surface a pattern of coloring matter corresponding to the pattern of the stencil. This stencil printing process is used to apply graphic images on many kinds of surfaces with many kinds of coloring matter.

Stencils may be prepared from paper, cloth, plastic film and other similar materials. They are cut, punched, etched taped or treated in some manner to produce a physical pattern on the sheet.

A typical sheet for making a stencil includes a plastic film support, for example, polyethylene terephthalate coated with a hydrophilic film-former such as gelatin. Gelatin has excellent properties for the preparation of stencils. However, it also has one serious disadvantage; it becomes brittle with age. Thus, in the form of a layer, aged gelatin develops cracks quite readily as the result of slight stresses. These cracks cannot be tolerated where the gelatin layer is used in a stencil since coloring matter would flow through and be deposited in unwanted areas.

Another serious disadvantage in the use of gelatin is the absorption of moisture from the atmosphere or from the fingers. In either case the gelatin becomes tacky and the stencil sheets become difiicult to separate from each other, or have no relative slip. Fingerprints show up and mar the quality of the stencil. Stacks of stencil sheets block or stick together. When the stacked sheets are pulled apart, pick-off or undesirable transfer of gelatin from one sheet to another occurs and the sheets are ruined.

Despite many attempts to overcome these problems, none were found to be satisfactory. Moistureproof packaging is sometimes unreliable and was found too costly; and a temporary expedient at best.

SUMMARY OF THE INVENTION In accordance with my invention and in spite of the recognized difficulty of protecting and preserving hydrophilic layers such as gelatin, I have discovered that such films or layers can be successfully protected and preserved with a special protective coating comprising a water soluble binder and an anti-tack and slip-promoting agent without interfering with their subsequent use or processing, for example, in the making of stencils.

Accordingly, one object of the present invention is to provide a novel and useful sheet which is preserved and protected from the effects of atmospheric moisture by a protective coating which does not interfere with subsequent processing.

Another object is to provide a new composition of matter for protecting and preserving a hydrophilic layer from the effects of atmospheric moisture without interfering with subsequent processing of the hydrophilic layer.

In accordance with my invention I have discovered that hydrophilic films or layers, such as gelatin, which are sensitive to atmospheric moisture can be successfully and effectively protected and preserved from the effects of atmospheric moisture without interfering with subsequent handling or processing of the hydrophilic film or layer in water or aqueous solutions by coating the film or layer with a protective coating material including a water-soluble, organic solvent-soluble binder in the solvent and fine, solvent-insoluble, substantially inert particles of an anti-tack agent uniformly dispersed in the protective coating material. In the case of a stencil sheet protected in accordance with my invention the sheet comprises a support carrying a hydrophilic or gelatin layer which in turn carries a dry protective coating of the binder material in which the particles of the anti-tack agent are uniformly dispersed.

For the purpose of this invention, I have found that a large number and variety of binders and anti-tack agents are effective for protecting and preserving the hydrophilic film or layer from the effects of moisture with out interfering with subsequent handling or processing of the hydrophilic film or layer in water or aqueous solutions.

Accordingly, the invention and its features are illustrated in the following examples without, however, intending to limit the scope of the invention.

EXAMPLE 1 Polyethylene terephthalate film supports were coated with gelatin from an aqueous solution to provide a number of units. The gelatin coatings on the supports were dried. A 5% by weight solution of hydfoxypropyl cellulose polymer as the binder in ethanol as the solvent, was prepared and 2 grams of zinc stearate powder, as the anti-tack agent, were uniformly dispersed in grams of the 5% solution.

This dispersion was then coated on the dry gelatin layers of one portion of the gelatin-coated units by the flow-down technique and then dried. A thin uniform protective coating was thus formed on the gelatin layer of each of these units. The thickness of the dried protective coating was about 0.1 mil.

The resulting protected gelatin coated sheets were then subjected to 70% relative humidity conditions at 70 F. for twenty four hours, after which the sheets were not tacky and handled with ease and free slip. Furthermore, the protectively coated sheets when stacked did not block or stick together. The sheets were easily separated with no pick-off.

By contrast, another portion of the gelatin coated units, NOT treated with the protective coating, were tacky to the touch under the same test conditions. The unprotected units stuck together in stacks and were separated with difficulty. Pick-off during separation ruined some of the unprotected units and they could not be used for making stencils.

The protected sheets may be used in various ways and in various processes, for example one of the protected sheets was sensitized by immersion in a 2% aqueous potassium dichromate solution for a few minutes. The agueous sensitizer solution penetrated the water-soluble protective coating and sensitized the gelatin layer. No special step or operation was required to remove the protective coating.

The sensitized stencil sheet was then exposed to ultraviolet light through a transparency. Where the actinic light struck the sensitized gelatin, a hardening action insolubilized the gelatin. The stencil sheet was then washed with warm water to remove unhardened parts of the gelatin layer which remained soluble in water. The protective coating was also washed off since it was not affected by exposure to light. Only the insolubilized areas of the gelatin layer remained on the film support. A relief image suitable for screen printing was then produced in the usual Way.

A Water-soluble dye may be incorporated into the gelatin layer to provide visual contrast in the relief image. The insolubilized areas retain the color of the dye while the washed out areas are colorless. This provides a visible indication of the completeness of the washing.

EXAMPLE 2 A polyethylene terephthalate film sheet support was coated with a gelatin solution as in Example 1, but in this case the gelatin solution included a dichromate sensitizer and a small amount of water-soluble dye. The coating was dried and then coated with a zinc stearate-hydroxypropyl cellulose dispersion the same as in Example 1. This protective coating was air-dried.

When subjected to 70% relative humidity at 70 F, for 24 hours, the protected sheet was tack-free and did not block when stacked with others similarly prepared. No pick-off occurred when the sheets Were pulled apart.

By contrast, unprotected presensitized sheets were tacky to the touch when exposed to the same conditions. The unprotected sheets stuck together when stacked, and pickoff occurred when the stacked sheets were pulled apart.

A protected sheet in accordance with this example was exposed to light through a transparency and washed in the usual manner to produce a relief image suitable for making a stencil. No special processing was required to remove the protective coating and at no time did the pro tective coating interfere with the normal functioning of the hydrophilic gelatin layer.

Thus, the protective coating protected and preserved the unsensitized as Well as the presensitzed layers Without interfering with normal processing.

EXAMPLE 3 A polyethylene terephthalate film support was coated with a gelatin solution as in Example 1. The gelatin layer was dried and then coated with 5% hydroxypropyl cellulose solution in ethanol into which solution slica powder was uniformly dispersed in the proportion of 2 grams to 100 grams of solution. The dry protective coating containing the uniformly dispersed silica powder protected and preserved the gelatin layer when it was exposed to humid conditions as in Example 1. Unprotected sheets were tacky under the same conditions.

Other anti-tack agents may be used in place of those in the above examples. Zinc oxide powder or titanium dioxide powder may be used. Other powders such as polyethylene powder and polypropylene powder are equally suitable anti-tack agents. Other metal soaps may be used in place of the zinc stearate but they must be fine, solid, dispersible powders. Suitable examples are the stearates of aluminum, magnesium, nickel and calcium, ferric and ferrous stearates zinc and aluminum palmitates. All of these materials are substantially inert, water insoluble and unreactive under the conditions of use.

Other water-soluble, solvent-soluble binders may be used. In addition to the hydroxypropyl cellulose, examples of other suitable binders are polyvinyl-pyrrolidone, polyethylene glycol, ethylene-maleic anhydride copolymer and polyethylene oxide. The suitability of other binder materials can be determined by simple tests. Suitable solvents include methanol and dimethyl formamide in addition to ethanol. Other organic solvents may be used and the solvent selected should dissolve the selected binder, and the combination should not affect the hydrophylic layer. The solvent should be vaporizable under the usual drying conditions.

Since the present invention is directed to an improvement, it will be understood that those skilled in the art will know the supports and the hydrophilic layer materials to be used, the effect of different grades of binder material on solution vicosity, the proper proportion of anti-tack agent to binder to be used for optimum results for each combination, and the dyes and sensitizers which are suitable.

Quite unexpectedly it was found that the incorporation of fine, dispersible, substantially inert and unreactive water and solvent insoluble particles into the protective coating effectively preserved the Water-soluble property of the coating while imparting tack resistance and slip to the protective coating. The presence of slip qualites is another unexpected benefit or result from the use of the coating.

It is speculated that the anti-tack agent Works and is effective under the conditions described because the solid particles of the agent protrude from the surface of the thin protective coating in sufficient numbers and over the Whole area of the sheet to eliminate tackiness due to the presence of atmospheric moisture. These particles may serve as many minute separators between sheets. In any case they cooperate with the binder to promote slip and eliminate tackiness. The ratio of anti-tack agent to binder is less than 1 to 1.

It is apparent from the foregoing description of the invention that many variations and modifications are pos-' sible, all of which are to be included within the scope of the present invention as defined by the following claims.

What I claim is:

1. In a sheet including a water-insoluble support and a hydrophilic layer coated on said support and normally affected by atmospheric moisture, the improvement comprising a protective coating on said layer for protecting and preserving said hydrophilic layer from the effects of atmospheric moisture without interfering in subsequent use and processing of said layer, said protective coating comprising a water-soluble and organic solvent-soluble binder, and fine, solid, water-insoluble and organic solvent-insoluble particles of an anti-tack agent uniformly dispersed in the coating in which the weight ratio of antitack agent to binder is less than one to one, said anti-tack agent being a metal soap substantially insoluble in water and unreactive with said binder and said layer.

2. In a sheet including a Water-insoluble support and a hydrophilic layer coated on said support and normally affected by atmospheric moisture, the improvement comprising a protective coating on said layer for protecting and preserving said hydrophilic layer from the effects of atmospheric moisture without interfering in subsequent use and processing of said layer, said protective coating comprising a waterand organic solvent-soluble binder and fine, solid waterand solvent-insoluble particles of an anti-tack and slip promoting agent uniformly dispersed in the binder of the coating, which particles are solid particles of a metal soap substantially insoluble in water and unreactive with respect to the hydrophilic layer, the weight ratio of anti-tack and slip promoting agent to binder being less than 1 to 1.

References Cited UNITED STATES PATENTS 2,833,671 5/1958 Funk et al 117-76 3,022,169 2/1962 Heckelmann et a1. 96-67 3,079,837 3/1960 Theilemann 88-195 GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

